Direct method for preparing doped polyaniline, product prepared thereby and resinous articles containing said product

ABSTRACT

Doped polyanilines are prepared by contacting aniline with an oxidizing agent such as ammonium peroxydisulfate in aqueous solution at a temperature of at most about 10° C. in the presence of at least one organic sulfonic acid, preferably an alkylbenzenesulfonic acid such as dodecylbenzenesulfonic acid. The products may be blended with thermoplastic resins such as polystyrene to produce blends having excellent static discharge properties

BACKGROUND OF THE INVENTION

[0001] This invention relates to conductive polymers, and moreparticularly to conductive doped polyanilines and compositionscontaining them

[0002] The use of conductive materials as additives for plastics is ofconsiderable interest since increased conductivity is desirable for manyapplications, including electrostatic painting and dissipation of staticcharges. For the latter purpose, especially, a very short static decaytime, on the order of 0.02 second, is desirable.

[0003] Among the conductive additives which have been employed areconductive carbon black and conductive carbon fiber. At high loadings,however, carbon black tends to accumulate at the surface of a plasticpart and subsequently slough off the plastic part. Carbon fiber isexpensive and can decrease impact resistance of the part.

[0004] It is also possible to coat the surfaces of particulatenon-conductive fillers such as mica, glass fibers or glass spheres witha conductive metal or compound thereof such as copper, silver orantimony-doped tin oxide. This, however, can cause problems includinghigh cost, insufficient adhesion to the base resin and loss inconductivity as a result of oxidation of the metal coating.

[0005] Many of these problems can be alleviated by employing aconductive polymer as the additive. A particularly advantageousconductive polymer is polyaniline, which may be prepared byelectrochemical methods or chemically by the oxidation of aniline in thepresence of a mineral acid under relatively mild conditions.

[0006] These preparation methods typically afford the polyaniline as apartially oxidized and partially reduced material, with the reducedportion comprising —C₆H₄NH— structural units and the oxidized portioncomprising alternating quinone and phenylene moieties separated bytrivalent nitrogen atoms. This partially oxidized and partially reducedpoltyaniline is known as “emeraldine”. It can be readily converted tothe fully oxidized state, known as “pernigraniline”, or to the fullyreduced state, known as “leucoemeraldine”. It can also be handled in theprotonated state, known as “protonated emeraldine”.

[0007] Polyaniline prepared by known methods, including theabove-described chemical method, has, however, various problems whichdetract from its utility as a conductive polymer. As a single example ofsuch a problem, such polyaniline is often contaminated with by-productssuch as the virulently carcinogenic benzidine.

[0008] PCT application 95/06,685 provides a summary of the state of theart of conductive polyaniline production and processing as of 1994,including the doping of polyaniline with such materials asalkylbenzenesulfonic acids. It also discloses a process in which dopedpolyaniline is combined with a polymer and a “solvent-plasticizer”toform a processable composition. Such a process requires a first step ofpreparing the polyaniline, a second step of doping it and a third stepof combining the doped material with a polymer and asolvent-plasticizer.

[0009] It remains of interest, therefore, to prepare a polyaniline whichis not contaminated with harmful by-products and which can be preparedin a minimum of steps and then blended with plastic materials to producea processable conductive material.

SUMMARY OF THE INVENTION

[0010] The present invention provides a single-step direct method forpreparing doped polyaniline and a method for separating the product fromthe reaction system I which it was prepared. The product of said methodis benzidine-free and has a static decay time which is substantiallyshorter than that of doped polyaniline prepared by other methods. Saidproduct can be combined in a single blending operation withthermoplastic polymers to afford processable conductive resinouscompositions.

[0011] In one of its aspects, the invention is a method for preparing anelectrically conductive polyaniline which comprises contacting anilinewith an oxidizing agent in aqueous solution at a temperature of at mostabout 10° C. in the presence of at least one organic sulfonic acid.Another aspect of the invention is sulfonic acid-doped polyanilinesprepared by this method.

[0012] A further aspect is conductive resinous compositions comprisingat least one thermoplastic resin and the sulfonic acid-dopedpolyaniline.

[0013] A still further aspect is a method of isolating polyaniline dopedwith at least one organic sulfonic acid which comprises precipitatingsaid doped polyaniline from aqueous solution by combination with amixture of water and a C₁₋₄ water-miscible alkanol and separating theprecipitated polyaniline from said solution.

DETAILED DESCRIPTION PREFERRED EMBODIMENTS

[0014] In the method of the invention for polyaniline preparation,aniline is contacted with an oxidizing agent suitable for its conversionto polyaniline. Suitable oxidizing agents are, in general, mild ones andtheir identities are known in the art. The preferred oxidizing agent,for most purposes, is ammonium peroxydisulfate, (NH₄)₂S₂O₈.

[0015] Also employed is at least one organic sulfonic acid. It may be analiphatic, aromatic or heterocyclic sulfonic acid; illustrative acidsare methanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid,3-carboxy-4-hydroxybenzenesulfonic acid and8-hydroxyquinoline-5-sulfonic acid. The preferred acids are thealkylbenzenesulfonic acids, most preferably those in which the alkylgroups contain about 4-20 carbon atoms; examples are2-n-octylbenzenesulfonic acid and 4-n-dodecylbenzenesulfonic acid, thelatter often being especially preferred.

[0016] Polyaniline preparation is effected by simply bringing theaniline into contact with the oxidizing agent and sulfonic acid at atemperature of at most about 10° C. at which the oxidation reactiontakes place, the preferred temperature range being about 0-10° C. Anaqueous medium is preferred. When the oxidizing agent is aperoxydisulfate such as ammonium peroxydisulfate, it is used in at leastan equimolar amount with respect to the aniline, preferably in a molarratio in the range of about 1.0-1.2:1. The sulfonic acid is preferablyused in the amount of about 5-10 ml per gram of aniline. Most often, theoxidizing agent is added gradually or portionwise to a mixture of water,aniline and sulfonic acid, and the resulting mixture is agitated tofacilitate polyaniline formation.

[0017] The polyaniline obtained according to the invention is alreadydoped with sulfonic acid and is generally in the form of a suspension inthe aqueous medium, said suspension being difficult to filter since thepolyaniline particles tend to clog filters of pore size less than about200 microns or pass through filters with larger pore sizes. According tothe polyaniline isolation method of the invention, the suspension iscombined with a mixture of water and a C₁₋₄ water-miscible alkanol suchas methanol, ethanol, 1-propanol, 2-propanol or 2-methyl-2-propanol. Thepreferred alkanol in most instances is 2-propanol, by reason of itsparticular suitability, availability and low cost.

[0018] Combination of the aqueous reaction mixture with thealkanol-water mixture is ordinarily effected under ambient conditionsincluding temperatures in the range of about 20-30° C. Saidalkanol-water mixture typically contains an amount up to about 95% byweight water with the balance being alkanol. Upon such combination, thedoped polyaniline precipitates as a fine powder which may be easilyremoved by filtration. After drying, typically in vacuum, it is readyfor use.

[0019] The entire process of polyaniline preparation and isolationaccording to the invention is most often completed in a period of 6-8hours. This is in contrast to the prior art method employing a mineralacid, which may require up to 10 hours for completion.

[0020] Sulfonic acid-doped polyanilines prepared and, preferably,isolated by the method of this invention are conductive and may be usedin the same way as other conductive polymers. In particular, it may beemployed to form a conductive coating on fillers and reinforcing agentsof high aspect ratio, such as glass fibers, which may then beincorporated in thermoplastic resins to increase their conductivity.

[0021] The doped polyaniline may also be used directly as a conductiveadditive for thermoplastic resins. Illustrative resins include additionpolymers, exemplified by olefin polymers such as polyethylene andpolypropylene; diene polymers such as polybutadiene, polyisoprene, highimpact styrene-diene copolymers and acrylonitrile-butadiene-styrenecopolymers; alkenylaromatic polymers other than diene polymers, such ashomo- and copolystyrenes; vinyl halide polymers; vinyl ester and alcoholpolymers; acrylic polymers including polyacrylonitrile, polyacrylamideand poly(alkyl acrylates) and poly(alkyl methacrylates) such aspoly(methyl methacrylate). Also included are condensation polymers,exemplified by polycarbonates such as bisphenol A polycarbonate,polyesters such as poly(ethylene terephthalate) and poly(1,4-butyteneterephthalate); polyphenylene ethers such aspoly(2,6-dimethyl-1,4-phenylene ether); polyarylene sulfides;polyetherimides such as the condensation product of2,2-bis[(4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride with atleast one of p- and m-phenylenediamine; and polyethersulfones. For manypurposes, polystyrene is a preferred thermoplastic resin. The resinousblends of the invention most often contain about 60-99% by weight of thethermoplastic resin and correspondingly about 1-40% of the dopedpolyaniline.

[0022] The invention is illustrated by the following examples.

EXAMPLE 1

[0023] A beaker was charged with 3,000 ml of water, 18.2 g of anilineand 90 ml of dodecylbenzenesulfonic acid. The mixture was cooled in anice bath to 10° C. and a solution of 45.2 g of ammonium peroxydisulfatein 200 ml of water was added dropwise over 1.5 hours, with stirring. Thereaction mixture was stirred for about 7 hours.

[0024] The product was an aqueous suspension of dodecylbenzenesulfonicacid-doped polyaniline which was found by gas chromatography-massspectroscopy to be free from benzidine. By contrast, a controlpolyaniline reaction mixture prepared similarly using hydrochloric acidin place of the sulfonic acid was found to contain a trace of benzidine.

EXAMPLE 2

[0025] A 400-g portion of the aqueous polyaniline suspension prepared inExample 1 was mixed with 400 g of water and 400 g of 2-propanol. Aprecipitate formed which was easily removable from the aqueous medium byfiltration through a filter with 1-micron pores. The filtration residuewas the desired dodecylbenzenesulfonic acid-doped polyaniline (11.11 g).

EXAMPLES 3-5

[0026] Blends of dodecylbenzenesulfonic acid-doped polyaniline, preparedand isolated by the method of Examples 1-2, and a commercially availablepolystyrene were prepared by mixing the resins in a Brabender mixer at220° C. for 5 minutes. The static decay times of the resulting blendsamples were determined by applying 5,000-volt positive charges theretoand measuring the time required for the charge to decrease to 500 volts.Three applications of positive charge were made to each sample at5-second intervals, followed by three similar applications of negativecharge.

[0027] The average positive and negative charge dissipation times foreach sample are reported in the following table. Comparison is made withtwo controls similarly prepared from the control polyaniline of Example1; i.e., the polyaniline prepared using hydrochloric acid andsubsequently doped with dodecylbenzenesulfonic acid. Avg. positive Avg.negative dissipation dissipation Example Polystyrene, % Polyaniline, %time, V time, V 1 98 2 0.02 0.02 2 95 5 0.01 0.01 3 90 10  0.01 0.01Control 1 98 2 6.56 2.37 Control 2 95 5 6.29 2.40

[0028] The results in the table clearly demonstrate the superiorconductivity of the doped polyaniline prepared by the method of theinvention, as compared with polyaniline prepared conventionally andsubsequently doped.

What is claimed is:
 1. A method for preparing an electrically conductivepolyaniline which comprises contacting aniline with an oxidizing agentin aqueous solution at a temperature of at most about 10° C. in thepresence of at least one organic sulfonic acid.
 2. A method according toclaim 1 wherein the oxidizing agent is ammonium peroxydisulfate.
 3. Amethod according to claim 2 wherein the sulfonic acid is analkylbenzenesulfonic acid.
 4. A method according to claim 3 wherein thealkylbenzenesulfonic acid is dodecylbenzenesulfonic acid.
 5. A methodfor preparing an electrically conductive polyaniline which comprisescontacting aniline with ammonium peroxydisulfate in aqueous solution ata temperature in the range of about 0-10° C. in the presence of at leastone alkylbenzenesulfonic acid, the molar ratio of said ammoniumperoxydisulfate to said aniline being in the range of about 1.0-1.2:1and the sulfonic acid being present in the amount of about 5-10 ml pergram of aniline.
 6. A method according to claim 5 wherein thealkylbenzenesulfonic acid is dodecylbenzenesulfonic acid.
 7. A sulfonicacid-doped polyaniline prepared by the method of claim 1 .
 8. A sulfonicacid-doped polyaniline prepared by the method of claim 3 .
 9. A sulfonicacid-doped polyaniline prepared by the method of claim 4 .
 10. Asulfonic acid-doped polyaniline prepared by the method of claim 5 . 11.A sulfonic acid-doped polyaniline prepared by the method of claim 6 .12. A method of isolating polyaniline doped with at least one organicsulfonic acid which comprises precipitating said doped polyaniline fromaqueous solution by combination with a mixture of water and a C₁₋₄water-miscible alkanol and separating the precipitated polyaniline fromsaid solution.
 13. A method according to claim 12 wherein thealkanol-water mixture contains an amount up to about 95% by weight waterwith the balance being alkanol.
 14. A method according to claim 12wherein the sulfonic acid is an alkylbenzenesulfonic acid.
 15. A methodaccording to claim 14 wherein the alkylbenzenesulfonic acid isdodecylbenzenesulfonic acid.
 16. A method according to claim 12 whereinthe alkanol is 2-propanol.
 17. A conductive resinous compositioncomprising at least one thermoplastic resin and a sulfonic acid-dopedpolyaniline according to claim 7 .
 18. A composition according to claim17 wherein the thermoplastic resin is selected from the group consistingof olefin polymers, diene polymers, alkenylaromatic polymers other thandiene polymers, vinyl halide polymers, vinyl ester and alcohol polymers,acrylic polymers, polycarbonates, polyesters, polyphenylene ethers,polyarylene sulfides, polyetherimides and polyethersulfones.
 19. Acomposition according to claim 18 wherein the thermoplastic resin ispolystyrene.
 20. A composition according to claim 18 comprising about60-99% by weight of said thermoplastic resin and correspondingly about1-40% of said doped polyaniline.
 21. A conductive resinous compositioncomprising about 60-99% by weight of polystyrene and correspondinglyabout 1-40% of a dodecylbenzenesulfonic acid-doped polyaniline accordingto claim 11 .